Bitumen is used as a binder in road asphalt mixtures, and has developed continually to meet ever increasing performance demands from road building constructors. In general bitumen performs well in road asphalt, but increasingly heavy traffic loads have led to the premature wear of many roads through rutting, ravelling (e.g., loss of aggregate material) and cracking of the surface. Asphalt binders nowadays often comprise a mixture of bitumen and polymer. Of these polymers, the thermoplastic rubbers forming a continuous network are the most commonly used polymers and have led to various papers on the subject matter.
For instance, in WO 97/44397 an asphalt binder is described comprising a bitumen component, a thermoplastic rubber in an amount of less than 8 wt. % and an ethylene-vinyl acetate copolymer in an amount of less than 5 wt. %, both based on total bitumen composition, wherein the ethylene-vinyl acetate copolymer has a vinyl content in the range of from 20 to 35 wt. %, based on copolymer. This application also describes a process for preparing the binder; and the use of the binder in asphalt mixes for road applications. This combination was specifically designed to improve the fuel resistance.
In EP 337282 A the asphalt binder comprises a combination of a linear styrene-butadiene-styrene block copolymer (SBS) having a styrene content of from 25 to 40% by weight and a melt flow index of from 1 to 15, and an ethylene-vinyl acetate copolymer (EVA) having a vinyl acetate content of from 40 to 50% by weight and a melt flow index of from 2 to 15, the SBS:EVA weight ratio being from 2:1 to 6:1. The product that has been used in this reference is Cariflex™ 1102, an SBS having an ordinary vinyl content (i.e., below 12 mol %, based on the total amount of butadiene in the midblock).
In EP 340210 A a composition for road surfaces and the like is disclosed that comprises from 85 to 97% by weight of bitumen and from 3 to 15% by weight of an additive component comprising 1) ethylene-vinyl acetate copolymers having a melt flow index of from 0.3 to 33 and a vinyl acetate content of from 15 to 45, ethylene polyacrylates, and 2) styrene-butadiene-styrene graft polymers having a styrene content of from 20 to 60% by weight, atactic polypropylene, rubber powder or mixtures thereof. The composition is characterised by a content of internal plasticizer/solvent from the group comprising ethylene-vinyl acetate copolymers having a melt flow index of from 33 to 500 and a vinyl acetate content of from 15 to 45, ethylene polyacrylates and/or naphthene-basic oils containing at least 55% of naphthenes, determined by the IR method, and having a maximum viscosity of 300 at 40 DEG C. A typical composition is given on page 5 of said reference, wherein 90% wt bitumen is combined with 2% wt SBS, 3% wt EVA 13-004, 1% wt EVA 18-150 and 4% wt plasticizing oil. Although both references EP 337282 and EP 340210 teach a combination of EVA and SBS, thus combining the advantages of both bitumen additives, further improvement remains desirable.
In MCKAY, et al. The Influence of Styrene-Butadiene Diblock Copolymer on Styrene-Butadiene-Styrene Triblock Copolymer Viscoelastic Properties and Product Performance. J. appl. polym. sci. (1995) May 23, vol. 56, no. 8, p. 947-958. the influence of a diblock on the properties of a triblock copolymer in an adhesive composition is described. This reference contains an evaluation of an SBS/SB blend in an Exxon Baytown AC-10 asphalt. According to the synopsis, the SB diblock copolymer quantitatively lowered the microphase separation temperature (MST) of the SBS triblock copolymer. These changes in linear viscoelastic behaviour manifest themselves into a reduction in the efficiency and performance of the SBS triblock copolymer in asphalt pavement binders and hot-melt adhesives. This reference hence clearly teaches away from using SB diblock copolymers. Also, no information is provided on asphalt binders with improved resistance to premature wear.
In WO 97/10304 an asphalt binder is described comprising a bituminous component and from 1 to 10% wt, based on the total bituminous composition of a block copolymer composition. The block copolymer composition comprises at least one of the group consisting of linear triblock copolymers, multi-armed block copolymers and diblock copolymers, which block copolymers comprise at least one block of monovinylaromatic hydrocarbon (A) and at least one block of a conjugated diene (B), wherein the block copolymer composition has a vinyl content of at least 25% by weight, based on the total diene content.
In the past, asphalt mixes were dense and often had a continuous grading of aggregate to reduce the voids volume. Nowadays, however, open asphalt and porous asphalt are increasingly used for pavements. Porous pavement is a special type of pavement that allows rain and snowmelt to pass through it, thereby reducing the runoff from a site and surrounding areas, but also reducing splash and spray from passing cars. Noise reduction is another highly desirable property of porous pavements. However, with their more open structure porous pavements have an even greater tendency of ravelling.
The present inventors therefore set out to find an asphalt binder that result in porous pavements with improved resistance to premature wear, and that may be applied in other asphalt mixes that require high durability and/or resistance to ravelling as well.